1. Field of the Invention
The present invention relates to methods and apparatus which can form a desired thin film such as a multi-layer optical thin film, an oxide thin film, etc., on a surface of any of various base materials, with good reproducibility.
2. Related Background Art
In conventional methods, conditions for obtaining a desired characteristic are determined, before formation of optical thin films by a trial and error procedure in which a film is formed on a monitor substrate that is identical to the base material on which the desired thin film is to be formed, an optical characteristic of the monitor substrate is measured by a spectrophotometer or the like, and computer simulation is conducted based on the results of the measurement. Further, the film formation on the monitor substrate and the measurement of the optical characteristic are periodically repeated carried out to check for change in the characteristic. If there is some change in the characteristic, calibration of the conditions is again performed to form a multi-layer optical thin film with the desired optical characteristic. Particularly in cases of film formation requiring high-accuracy thickness control, as in forming an optical thin film, the monitor substrate is set at a position where the film is formed in a thickness nearly equal to that on the production substrate and, monitoring the thickness of the film by optically measuring the thickness of the film deposited on the monitor substrate, the thickness of the film deposited on the production substrate is controlled. Available methods for monitoring the film thickness include use of a quartz oscillator film thickness monitor which can also measure the thickness of opaque films, and a method for monitoring the rate and thickness by discharge spectral analysis of the film materials, as well as the above optical means.
For forming a thin film by sputtering, the sputtering rate is arranged to be kept stable by keeping constant parameters associated with the sputtering rate, e.g., by monitoring an ultimate pressure before film formation, controlling purity or a flow rate of introduced gas, monitoring the sputter power, controlling the purity of the target, controlling the temperature or flow rate of cooling water, etc.
Thin film formation utilizing the sputtering phenomenon (hereinafter referred to as sputtering) is widely utilized because the film can be relatively easily formed with high-melting-point materials or compounds not suitable for vapor deposition, and because the film can be readily formed over a large area. In such sputtering, heating (baking) of the vacuum chamber is normally conducted to reduce the amount of gas released during sputtering from the walls of the vacuum chamber and other components, in order to improve the uniformity of the film and the efficiency of the film formation. Apparatus for performing such sputtering employ, instead of the conventional batch processing, a method for suppressing the release of gas during sputtering so as to improve the quality of the vacuum in the vacuum chamber, as in single-wafer-processing sputtering apparatus, inline sputtering apparatus, etc. provided with a prechamber (load-lock chamber).
General methods for producing an oxide thin film utilizing plasma are the sputtering method, the vapor growth (CVD) method, and the ion plating method. However, if these methods are used to produce a normal thin film, a composition ratio of the film is affected because of insufficient oxidation of the thin film, or because of atoms being deposited without being perfectly bound in the film. They could be a cause of optical absorption. When such optical absorption occurs in the thin film, the transmittance is decreased for an anti-reflection coating formed of the thin film, or the reflectivity is decreased for a high-reflection film, whereby the desired functions of the optical thin film are lost. The tendency for this to happen increases as the wavelength of the employed light decreases. Also, the absorption of light heats an absorbing portion, which could damage the film itself or distort the ground substrate, lowering surface accuracy. Therefore, the optical absorption must be reduced as much as possible. Conventionally, the substrate has been heated during or after the film formation to promote oxidation on the substrate and to get rid of unbonded atoms.